Is this a good method of checking if pool is leveled?

[LysergicBliss]

I took a 4ft level and noted the waterline depth in each corner. (Inside the pool, from the ground up)
One corner was 88cm high, the closest adjacent corner was 90cm. (Width side)
The opposite corner of that one was 91cm (length side) and the corner opposite to that one was 89cm. (width side).
In other words, the length side appeared to be off level by 1cm and the width side seemed to be off level by 2 cm.
I did this a few times as it was filling up and the differences were always 1cm and 2cm.

Here is what is confusing me, if you look at the liner, the pool looks to be off level by about 2 inches. Measuring down from the top of the pole to where the water line starts. What is the proper way to measure if the pool is leveled? Is my pool un level by 1(length)/2(width) cm's or 1(length)/2(width) inches?

 

[midtempo]

You can use a bubble level on the rails.

 

[Krocko]

Sounds like the liner may be uneven if the water level measurements are on